1. Field of the Invention
The present invention relates to a radio channel allocating system, in particular, to a radio channel allocating system for use with a digital communication system for allocating radio channels to a plurality of cells (radio zones) into which a service area is divided.
2. Description of the Related Art
In a conventional mobile communication system, a cellular system of which a service area is divided into a plurality of cells (radio zones) for effective use of frequencies has been used such as preventing interferences and disturbances.
Line designing methods of which frequencies are allocated to cells in such a manner that the same frequencies are reallocated to other cells that are spaced apart for a predetermined distance without interferences of the frequencies have been proposed and accomplished as, for example in (1) Victor Graziano, "Propagation Correlation at 900 MHz", IEEE Trans. on VT. Vol. VT-27, No. 4, 1978, (2) J. E. Stjernvall, "Calculation of Capacity and Co-channel Interference in A Cellular System", Nordic Seminar, pp. 209-217, Espoo, Finland, 1985, (3) J. J. Mikulski", Dyna TAC Cellular Portable Radio Telephone System Experience in the U.S. and the U.K.", IEEE Comm. magazine, Vol. 24, No. 2, 1986, and so forth.
As described in the above technical papers, cells (radio zones) are categorized as a triangular zone (shown in FIG. 9(a)), a square zone (shown in FIG. 9(b)), a hexagonal zone(shown in FIG. 9(c)), and so forth, corresponding to the shape of the cells. FIG. 10 shows the relation among the number of repetitive zones and the ratio of the distance of an interfering station at the worst interference point and the distance of a desired station (see Okumura et al, "Fundamentals of Mobile Communication (written in Japanese)", edited by The Institute of Electronics, Information, and Communication Engineers, published by Korona-sha, 1986). In FIGS. 9(a) to 9(c), numerals represent repetitive frequency patterns and repetitive zones.
As countermeasures for alleviating radio interferences to other cells, a system for mechanically or electrically tilting a main beam on the vertical plane of an antenna of a base station for reducing the interferences has been proposed (as in Fujii et al, "Reducing Interference on Same Channel in Mobile Communication by Antenna Tilting (written in Japanese)", 1990 Autumn National Convention of The Institute of Electronics, Information, and Communication Engineers, b-247, 1990).
When a plurality of signals are input to a non-linear circuit such as a transmitting unit or a receiving unit, a signal with a third frequency other than frequencies of the input signals (namely, a intermodulation wave) are produced by the plurality of signals. For example, when the frequency of the intermodulation wave that products in a receiving unit of a base station or the like due to a signal with a high input level accords with the frequency of a signal with a low input level generated by a child station or a mobile station, an intermodulation interference takes place.
In the conventional cell reuse system and the tilt antenna system, they describe that only interferences that take place in other cells are prevented. In other words, an interference due to a intermodulation wave that takes place in the same cell cannot be prevented.
A radio channel allocating system that considers the influence of a intermodulation wave that takes place due to a radio channel allocation has been proposed by W. C. Babcock, "Intermodulation Interference in Radio Systems," Bell System Technical Journal, 32, l, pp. 66-73, 1953. However, in the conventional radio channel allocating system, an interference to a normal signal in the case that the frequency of a intermodulation wave accords with the frequency of a signal received by a base station in a particular zone is considered. In this system, frequencies for individual zones are selected so that a particular condition is satisfied. However, an influence of which a mobile station incorrectly receives a intermodulation wave with a frequency that is not used in the zone as a wave transmitted by a base station is not considered.
Moreover, the conventional radio channel allocating system does not have countermeasures of which cells are multiplexed corresponding to the distance from a base station and channels are repeatedly allocated to inner cell parts and outer cell parts.
Furthermore, in the system of which channels with the same frequencies are multiplexed by time division multiplex accessing method for increasing the accommodating ratio of subscribers, interferences against guard slots are not considered. Thus, a main signal wave is adversely interfered with a intermodulation